Stringent criterion reassociation will be used in the further exploration of genome arrangement and of genome rearrangement in evolution. Interspersion curves and S1 curves combined with agarose chromatography will be used in these determinations. Our method for polysomal mRNA isolation will be developed further. Polysomal mRNA will then be used in the isolation of expressed sequences. These sequences in turn will be tagged with poly A "tails" and they will subsequently be used to determine relative rates of evolution of expressed sequences and to isolate a class of sequences common to several different organisms, if present. These studies may then be extended to an examination of the relative rate of evolution of expressed sequences and their repetitious sequence partners.